Defending the Planet: NRC Final Report

byPaul GilsteronJanuary 22, 2010

I’m looking at the National Research Council’s final report on the detection of near-Earth objects, the culmination of the study that produced the NRC’s interim report last year. Let’s recall the context: It was in 2005 that Congress mandated that NASA find 90 percent of NEOs with a diameter of 140 meters or greater, such discovery to be concluded by 2020. The interim report, discussed in an earlier Centauri Dreamsstory, concluded that NASA couldn’t meet this goal because funds for the survey had never been appropriated.

Now we have a final report with suggestions on what NASA could do to finish the survey as soon as possible after the original 2020 deadline. Two possibilities emerge: A space-based telescope working in tandem with a ground-based telescope could finish the job the fastest. But if cost-cutting is necessary, the space option will have to be abandoned in favor of ground-based equipment. Gratifyingly, the NRC stands up strongly to defend Arecibo, whose role in asteroid observation is crucial, and also voices strong support for radar observations at the Goldstone Deep Space Communications Complex. From the report:

…the complementary radar systems at Arecibo and Goldstone are powerful facilities for characterization within their reach in the solar system, a maximum of about one-tenth of the Earth-Sun distance. Arecibo, which has a maximum sensitivity about 20-fold higher than Goldstone’s, but does not have nearly so good sky coverage as Goldstone, can for example, model the three-dimensional shapes of (generally very odd-shaped) asteroids, and estimate their surface characteristics, as well as determine whether the asteroid has a (smaller) satellite or satellites around it, all important to know for planning active defense. Radar can also accurately determine orbits of NEOs, from a few relatively closely spaced (in time) observations, which has the advantage of being able to quickly calm public fears (or possibly, in some cases, show that they are warranted).

Thus this recommendation:

Immediate action is required to ensure the continued operation of the Arecibo Observatory at a level sufficient to maintain and staff the radar facility. Additionally, NASA and NSF should support a vigorous program of radar observations of NEOs at Arecibo and NASA should support such a program at Goldstone for orbit determination and characterization of physical properties.

Image: Renderings of binary near-Earth asteroid 1999 KW4 showing its satellite making one orbit. The figure shows three-dimensional models in shaded relief, reconstructed from a set of radar images obtained at Arecibo and Goldstone in 2001. The models are shown in their proper orientation as viewed from Earth. Radar imaging has shown that ~15 percent of NEOs larger than 200 meters in diameter have one (or sometimes two) satellites. SOURCE: S.J. Ostro et al., Radar reconnaissance of near-Earth asteroids, pp. 143-150 in Near Earth Objects, Our Celestial Neighbors: Opportunity and Risk (A. Milani et al., eds.), Proceedings of the 236th Symposium of the International Astronomical Union, Prague, Czech Republic, August 14-18, 2006, Cambridge University Press, Copyright 2007 International Astronomical Union.

The asteroid or comet that struck the Yucatan some 65 million years ago is estimated to have been about ten kilometers in diameter, the kind of object that strikes the Earth only once every 100 million years on average. An object 140 meters in diameter, the size mandated by the survey requested of NASA, would cause regional damage, with impacts happening on average every 30,000 years, according to the report.

What to do if an NEO is discovered on a collision course with the Earth? The alternatives include using a ‘gravity tractor’ or other ‘slow push’ methods to move the object, but these are more practical for smaller objects (up to 100 meters in diameter) and would require decades of warning time. Changing a NEO’s orbit by flying a large spacecraft into it might also require warning times longer than we would necessarily have. Larger NEOs (with diameters greater than one kilometer) would demand the nuclear option:

…current technology allows us to deliver payloads for mitigation to NEOs in a wide range of orbits. However, in cases of short warning (under, say, a decade), payloads are likely to be severely limited in mass, but may often be sufficient to deliver a nuclear device. The development of the next generation of heavy-lift launch vehicles will considerably improve the situation. The development of advanced engines for in-space propulsion will considerably improve our capability for delivering rendezvous payloads (for characterization, to act as gravity tractors, or to emplace surface explosives) when the warning time is decades.

Image: Approximate outline of the regimes of primary applicability of the four types of mitigation (see report for the many caveats associated with this figure). Image Courtesy of Tim Warchocki.

The problem we face is that while these methods may work on paper, we’re not ready to put any of them to work on short notice. We also lack basic information about the kind of target we would be trying to deflect or destroy. Because of this, the report recommends a new program dedicated to more fully dealing with the hazards of NEOs:

The United States should initiate a peer-reviewed, targeted research program in the area of impact hazard and mitigation of NEOs. Because this is a policy driven, applied program, it should not be in competition with basic scientific research programs or funded from them. This research program should encompass three principal task areas: surveys, characterization, and mitigation. The scope should include analysis, simulation, and laboratory experiments. This research program does not include mitigation space experiments or tests which are treated elsewhere in this report.

We’re left, too, with the question of how small an NEO can be before we safely disregard it. The 1908 impact at Tunguska devastated more than 2000 square kilometers of forest. And while earlier estimates for the size of this object have been around 70 meters in diameter, there is recent research indicating the object could have been as small as 30 meters in diameter. That conclusion is, the report notes, preliminary, but because smaller objects are more numerous than larger ones, the likelihood of such an impact event is approximately once every three centuries. We also need to learn more about airbursts from impactors in this size range and whether they can cause tsunamis that damage coastlines.

The recommendation:

Because recent studies of meteor airbursts have suggested that near-Earth objects as small as 30 to 50 meters in diameter could be highly destructive, surveys should attempt to detect as many 30- to 50-meter objects as possible. This search for smaller-diameter objects should not be allowed to interfere with the survey for objects 140-meters in diameter or greater.

Space missions like NEAR Shoemaker and Hayabusa have given us information on two NEOs, and what we’ve learned is that Eros and Itokawa are dissimilar in many ways. From these missions and ground-based observations, we’ve learned that NEOs have a wide range of internal structures and more complex surfaces than had been realized. It’s clear that we need to learn more about these properties through further dedicated space missions if we’re to develop the strategies that will ensure we can move or destroy a NEO.

You can get a copy of the NRC report “Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies” through the National Academies Press. It’s absorbing reading as we consider the problem of how to deal with a threat that may be exceedingly rare but potentially catastrophic. Our technology is only now getting to the point where we have the capability of affecting the trajectory of an incoming asteroid, depending upon its mass and the amount of advance warning we have. My belief is that a space-based infrastructure developing over the next century will come to play an essential role in safeguarding our planet.

Since this research concerns the safety of the entire planet shouldn’t all nations (not just the US) contribute to the finance for research and development to NEO deflection? There should be a department in the UN to manage all this for eg.

“The 1908 impact at Tunguska devastated more than 2000 square kilometers of forest. And while earlier estimates for the size of this object have been around 70 meters in diameter, there is recent research indicating the object could have been as small as 30 meters in diameter. That conclusion is, the report notes, preliminary, but because smaller objects are more numerous than larger ones, the likelihood of such an impact event is approximately once every three centuries.”

Can you point me to the research that give a mean rate of once every year? The information I have suggests a Tunguska event every 1900 years and a 30m impactor every 7 or 8 hundred years.

I am reasonably confident that Tunguska will turn out to be the last asteroid/comet strike to have had the potential for massive loss of life. Why? Because I think we’re well on the way to developing the capacity to detect all such dangers from above. Sure, it might take several more decades to complete the job, but with every year that passes, the odds of us being caught by surprise are being lengthening considerably.

In the long run, were going to have far more trouble with what’s below our feet than the stuff above our heads. Just in the last few years we’ve had devastating earthquakes in China, the Indian Ocean (tsunami) and now Haiti, and there is precious little we can do to mitigate the problem without spending uncountable hundreds of billions of dollars earthquake-proofing buildings in all the major quake prone areas, and that’s just not going to happen. Far more likely is that rapidly increasing populations in these area are going to lead to far greater death tolls in the future.

And that’s just for starters. We’re centuries away from solving the problem of caldera volcanoes and massive underwater landslides (that cause tsunamis that would dwarf the Indian Ocean tsunami), and that’s if we ever will be able to stop them, or even mitigate their effects beyond an early warning system and the mass evacuation of hundreds of millions of people.

Work by Boslough and Crawford (1997; 2008), however, indicates that a much lower yield could produce the same effects. They found that asteroid airbursts do not act like point explosions in the sky (e.g., like a nuclear bomb explosion) but instead are more analogous to explosions along the line of descent. In an airburst, kinetic energy (see Appendix E) is deposited along the entry path, with significant downward momentum transferred to the ground. Accordingly, they suggest that smaller explosions with net yields of 3 to 5 MT may be sufficient to produce Tunguska-like impact events. If true, the average interval between Tunguska-like events using the Harris (2009) size distribution (see Figure 2.4) would be on the order of a few hundred years. These results would increase the calculated hazard from smaller objects, perhaps as small as 30 meters or so. Further research is needed to better characterize this threat.

The Harris reference is to Harris, A.W., The NEO population, impact risk, progress of current surveys, and prospects for future surveys, Presentation to the Survey/Detection Panel of the NRC Committee to Review Near-Earth Object Surveys and Hazard Mitigation Strategies, January 28-30, 2009. Some of this data will also be published in an upcoming European Space Agency conference proceedings of the April 27-30, 2009 1st IAA Planetary Defense Conference: Protecting Earth from Asteroids.)

I do wonder if Tunguska wasn’t a dark matter object – Robert Foot’s Shadow/Mirror matter research investigated the possibility of a small interaction between regular and mirror matter which allows some regular matter to be caught up in the shadow matter and the creation of a shockwave around an infalling mirror matter meteor. Thus no (definite) Tunguska crater, yet destructive effects from a re-entry shockwave. Would be incredible to find a chunk of it – invisible yet we could feel its weight.

Denver, IIRC, Soviet Research showed that the trees fell in an overall ‘butterfly’ pattern that showed the explosive/kinetic energy released over a ‘small’ portion of it’s final fight path. One of the ways they figured the bolide’s flight path through the Earth’s Atmosphere.

I am slightly familiar with this research, now that you have jogged my memory. If I recall correctly the Soviets showed that the “butterfly” pattern was common to air bursts from bombs and artillery shells.

Interestingly, the butterfly pattern also resembles the highly lobate ejecta patterns seen in young crater forms.

This reminds me of visiting the Meteor Crater in Arizona as a kid. According to wiki, it was caused by a 50-meter-wide impactor which hit the earth 40,000 years ago.

Overall, we’ve been pretty lucky when it comes to meteors. As I’ve heard others say, we live in a cosmic shooting gallery. The earth does have advantages though – our atmosphere helps burn up asteroids, jupiter and saturn can capture them or pull them off course, and the moon catches them too – as evidenced by its numerous craters, especially on the dark side.

Still, the possibility is out there, so we need some sort of detection system that will give us at least a reasonable window of time. I’m confident that nuclear weapons would do well in deflecting asteroids off of their collision course so that they’d miss the earth.. or perhaps sent into a falling orbit that would burn them up in the atmosphere.

I recently created a documentary for the Space Generation Advisory Council on these very issues from interviews filmed at the above mentioned 2009 Planetary Defence Conference in Grenada. It features Boslough, Harris and others talking candidly about what they think of the NEO threat and also about how close we are to the point where we can deal with them.

It is funny, that the response graph does not contain other possible responses ( a cynic would say, the more probable ones, but I am not a cynic ). Like “Yay ! Apocalypse!”, “Just not the dreaded N thing, PLEEEASE!”, “Asteroid impact denialism”, “don’t you dare to interfere with the the divine punishment”, “Pray/repent your sins!”, etc.

For really nasty jobs that need to be accomplished on fairly short order, I am in favor of a 1 megaton yield robust nuclear Earth penetrator type device. Given that even pure diamond which has about as high of a heat of vaporization and atomization as any materials, at about 710.9 kJ mol-1 = 59.24 x 10 EXP 6 joules per kilogram, a one megaton nuclear device has a yield suitable to completely vaporize a maximum of 67,500 metric tons of diamond or melt about 409,960 metric tons of pure diamond when considering the heat of fusion of diamond at 117 kJ mol-1.

A ten megaton device can completely vaporize 675,000 metric tons of diamond and completely melt 4,099,600 metric tons of diamond which is the equivalent of a 105.21 meter cube of diamond or a cube of diamond with an edge length of 345.177 feet.

A 100 megaton nuclear device can in theory vaporize a maximum of 6,750,000 tons of diamond which works out to a 124.24 meter cube or a cube of pure diamond that is 407.6 feet in edge length. The device could completely melt 40,099,600 metric tons of diamond which is the equivalent of 11,392,000 cubic meters of diamond. This works out to the equivalent of a 225 meter wide cube of diamond or a 738.22 foot wide cube of diamond.

If pure fusion nuclear devices are possible that achieve a near 100 percent burn-up rate, then robust penetrators that contain 10 metric tons of fusion fuel and 15 metric tons of hardware including the devices armored cladding can in theory have a yield of up to 1.75 Gigatons or 1,750 megatons. I would much prefer to unleash 1,750 megatons of pure fusion energy which would be much cleaner than a somehow produced fission bomb, or more likely a fission-fusion-fission bomb of the same yield.

This device could completely vaporize a 450.6 meter wide cube of diamond and completely melt a 888 meter wide cube of diamond.

In reality if the device was detonated in the middle of an asteriod, the kinetic energy of the blast could tear the asteriod apart and cause high velocity seperation of its fragments while vaporizing and atomizing the rest of the molten asteriod by the mechanical blast energy.

I choose diamond because of its high end limiting heat of vaporization and heat of fusion. In reality, many asteriods of the above exemplar volumes could be vaporized and/or melted with much lower yield devices because of the lower effective heats of vaporization and fusion of their composition.

@James, this is not the desired mechanism for asteroid impact mitigation. Indeed, for most asteroids, vaporization is not necessarily possible (the result of a deep-impact nuclear explosion would be fragmentation, in almost all cases), nor necessarily desirable. Rather, nuclear weapons would be used for propulsion in diverting an asteroid off an impact course. In vacuum a nuclear explosion is primarily a very strong soft x-ray lightbulb, these x-rays can be used to ablate a thin layer of material from a large swath of the asteroid’s surface. This ablation would generate a not inconsiderate thrust, which, with a number of applications dependent on lead-time, should be enough to move the asteroid off of an impact course.

I once read of the concept of using a 1 gigaton neutron bomb to ablate a comet or an asteriod in the manner simmilar to what you recommend as possible above.

Now given that the neutron bomb designs were generally on the order of one kiloton, it might take some doing to scale the device up to one gigaton yield. Perhaps a blanket array apparatus composed of a sheet of low yield neutron bombs connected by cables or other mechanisms could be assembled in space and flown on an intercept course with the asteroid.

All of the neutron bombs in the array would be detonated at a precisely pretimed and/or simultaneous manner in a standoff blast so as to irradiate the outer surface of the asteriod to the point where the surface vaporizes.

If nuclear devices are used that produce a large percentage of their energy in hard radiation such as gamma rays, neutrons, fusion products etc, then I have great hope for your scenario.

Note that the 10 megaton test of the first full scale hydrogen bomb left a crater about 7,000 feet wide and 280 feet deep in the Atoll island where it was detonated. Granted that coral is not the mechanically most blast resistant material, but I sure it was highly water logged and as a result, much of the ejected portion of the creater was likely vaporized despite the high heat of vaporization of water. Simply put, ten megatons is enough to vaporize 20 million metric tons of water, although most of the above reference H-bomb’s energy went into the atmosphere without being coupled to the surface below its point of detonation.

However we can work out the appropriate yields and other factors, I think nuclear devcies can be of use in diverting asteriods.

James mentions a nuclear device delivered with a penetrator. Depending on how deep that goes, it will do a lot more than ablate a thin layer of the surface.
It will melt/vaporize it from the inside out.
The question is, of course, how deep can you get. Rubble pile asteroids should be easier than solid metal ones to penetrate.

A new report last week suggested that near Earth object survey efforts require significant additional funding. Jeff Foust examines why the relative lack of money so far may in fact be a rational decision, and what could be done to improve their funding prospects.

It seems to me the most efficient way to use a nuclear device would be to let it penetrate just a few meters, enough so most of its energy is used to push rock, rather than being dispersed into empty space. In the very center, we would get unpredictable fragments, but when still near the surface we would be pretty much guaranteed that there would be two components, a volley of ejecta going in one direction and the rest of the asteroid into the other. With the proper choice of impact point, it should be fairly easy to get both safely out of the way.

I agree with your above comments. As some one who enjoys contemplating really big fire crackers as times (the little boy in me who liked to play with firecrackers perhaps is still thier), I have contemplated possible theoretical mechanisms of essentially literally unlimited yield nuclear devices. The bigger the nuke, the larger the yield and the more material that such a device could vaporize.

Eniac, I see the validity of your points as well. It has been well known by folks who study nuclear targeting of deeply buried hard targets such as bunkers that a sub-surface blast of even a depth of few meters can greatly increase the mechanical blast energy coupling to the Earth’s crust and enable the destruction of very deeply buried targets with nuclear devices of modest yield.

I still feel what whatever nuclear scenario would be most appropriate for a given scenario, nukes may be our best option for hitting an asteriod on relatively short notice.

Another cool idea is to attach a huge nuclear thermal rocket onto the asteriod and perturb or deflect its orbit so that it drops into the Sun thus eliminating the threat.

James, you can deflect an asteroid enough to keep it from hitting Earth with a delta-v of mm/s or less, I believe. To drop one into the sun would require tens of km/s, a MUCH harder (strenuously avoiding the word impossible) task and absolutely not worth the extra effort.

Moreover, deflecting an asteriod little by little, if it is of Earth orbit crossing trajectory might be usefull for mineral aquisition by mining the asteriod. Some asteriods may have rich deposits of precious and exotic elements.

The only way I could forsee a nuclear thermal rocket pushing say a 1 billion metric ton asteriod into the Sun is perhaps by utiliizing a very large rocket and a tank of propellent containing roughly the same order of magnitude mass as that of the asteriod: probably not much worth the effort. Besides, we might throw away a potentially valuable source of minerals and metals. This assumes an Isp of about 1,000 seconds to 2,000 seconds for the nuclear thermal rocket. I think, if I am not mistaken that the NERVA engine developed only at most and Isp of around 1,000 seconds. As for nuclear electrical propulsion systems, they might work with a much smaller quantity of fuel, but so far ion rockets have a very low thrust. An amped up VASIMR rocket might work here.

Now developing very large scale fusion rockets would definately be applicable here when they are eventually realized.

Personally, I think the best method of hauling asteroids (more than just a simple nudge to avoid collision) is to install a large number of nuclear power plants and mass drivers on it. You then use this equipment to expel a large fraction of the asteroid as propellant.

The best mass drivers, I think, would be rotating slings with a tip velocity as high as you can get, with existing materials perhaps 3-4 km/s. In quick succession, you attach pieces of asteroid in the middle, and let them slide out centrifugally, where they will slip off the tip at a velocity of sqrt(2) times the tip velocity, maybe 5 km/s. You keep the sling from slowing down using a powerful electric motor at the hub. Mechanically, the whole thing does not need to be more elaborate than a carnival ride, really, except for the long and strong tether. A good sliding mechanism that is easy on the tether and works at high velocity might be a block of ice, or a gas cushion.

With this method, you could realistically achieve many km/s of delta-v, and still keep a sizable fraction of the asteroid at the end. It should be enough to move the asteroid into Earth orbit (not that we would want to try such a thing), but probably a stretch to send it into the sun (ditto).

None of this is of any practical importance, I think, as the tale of Mohammed and the mountain applies here: It is much better to go to the asteroid than to try and fetch it. Given the price of even a single, non-spaceworthy nuclear power plant, the above method is hideously expensive for reasonably sized asteroids. Besides, if you wanted a pile of rocks in Earth orbit, all you’d have to do is go to the moon. For all these reasons, the above scenario will not likely ever (strenuously avoiding the word never) be economical.

However, if you think one solar powered sling instead of thousands of nuclear powered ones, and payload instead of propellant, you get a very cheap transportation system for raw materials extracted from the asteroid, so not all is for naught. And slings would work on the moon, as well.

The future looks suddenly bright for Arecibo, the world’s largest single-dish telescope. Brutal funding cuts in the last few years threatened to eliminate the Cornell-operated observatory completely. But a new report filled with glowing praise raises hopes that Arecibo will continue to focus on the skies.

My question is How come we didn`t know about the Meteor that barely discepated over the Solviet Union, I taught that the HARP ARRAY was our early warning system and was design to take down such threats.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last eleven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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